diversity - latitudinal trends diversity · equilibrium hypothesis for latitudinal gradients in...
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Temperate forests Tropical forests
# species:
Trees (per 10 ha) 15 450-700Birds 20-30 >300
Insects many millionsFish many 4 timesAnts entire Utah = 1 tree
Diversity - latitudinal trends
For most groups of organisms, the tropics is 10-100 times more diverse
Diversity
1. Higher latitudes have lower diversity (data from N. America)
trees
birds
mammals
Diversity
2. Higher altitudes have lower diversity
Birds
Mammals
Plants
Low - altitude - Hi
3. Islands have lower diversity
How do we measure diversity?
a. Species richness: total number of speciesb. Species evenness: how evenly are the individuals distributed
among species? [takes into account abundance]
A B C D E F A B C D E F
community A community B
# in
div
idu
als
(o
r b
iom
ass)
Two hypotheticalcommunities with
the same speciesrichness butdifferent species
evenness
Scales of diversity?
a. Beta (!) diversity: diversity across habitats
(mountains, valleys, swamps ….)
diversity can be high because there are lots of different habitats
b. Alpha (") diversity:
diversity within asingle habitat
diversity can be high ifmore species cancoexist in the same
habitat
Higher alpha diversity reflects niche partitioning
Resourc
e u
se
Resource (seed size, temperature, etc…)
Niche overlap
niche breadth {
1. Narrower niches
2. Greater overlap
3. Extend theresource axis(greater range of
availableresources)
So can we explain the latitudinal gradient in diversity using factors
that would affect niche partitioning?
2
1. Higher productivity will lead to higher diversity
a) longer axis - more species can co-occur if using extremeends of resource axis
Equilibrium hypothesis for latitudinal gradients in diversity:(assume communities are at their equilibrium number of species)
1. Higher productivity will lead to higher diversity
a) longer axis - more species can co-occur if using extremeends of resource axis
b) species can be more specialized and have narrower niches
Equilibrium hypothesis for latitudinal gradients in diversity:(assume communities are at their equilibrium number of species)
1. Higher productivity will lead to higher diversity
a) longer axis - more species can co-occur if using extremeends of resource axis
b) species can be more specialized and have narrower niches
c) species can have more niche overlap
The tropics are more productive (warm and wet year-round)
Greater plant productivity could lead to greater plant diversitywill mean greater animal diversity
Equilibrium hypothesis for latitudinal gradients in diversity:(assume communities are at their equilibrium number of species)
High resources
Low resources
Equilibrium hypothesis for latitudinal gradients in diversity:
2. Climate or resource stability will lead to higher diversityAt high resource levels, species A, B and C can coexist and D will be
excluded, but at low resource levels A, B and C will go extinct. Only a generalist(D) will have sufficient resources
D
- So variation in resources will favor increased niche breadth- Species with too much overlap will be out competed during low resource
periods
- the tropics are generally more stable than the temperate zone
Non-equilibrium hypothesis for latitudinal gradients in diversity:
(assumes that communities have not had enough time for diversity toreach maximum levels for that habitat)
Ecological time: communities may be younge.g. there has not been enough time since a recent disturbance forspecies to recolonize the habitat.
FireFloodLandslideAbandoned agriculture ….
Non-equilibrium hypothesis for latitudinal gradients in diversity:
Evolutionary time: communities may not have had sufficient time oropportunity for speciation (but they could hold more species)
1. The tropics have remained near the equator for long periods oftime while the temperate habitat is newer (tectonically)
2. The temperate zone has hasless time since glaciations
3. Rates of speciation may differin different habitats. Are theyfaster in the tropics? (yes but
under debate)
4. Rates of extinction may differ
in different habitats. Are theyslower in the tropics? (yes butunder debate)
3
Human impact on biodiversity
Before humans arrived in North America about 12,000 yrs ago,what did the fauna look like?
Enteledont feeds on rhinoceros
Mastodon
Human impact on biodiversity
Before humans arrived in NorthAmerica about 12,000 yrs ago,what did the fauna look like?
Titanus (3m, 400Kg predatorybird)
Human impact on biodiversity
Major extinctions of megafauna occurred 12,000 yrs ago
Extinct species include: lion, bison (different species), mastodon,mammoth, dire wolf, short-faced bear, giant ground slot, saber
tooth cat, Titanus (3m, 400Kg predatory bird) …
North American survivors: ONLY mule deer and black bear
All other mega-mammals present in N. America today colonizedfrom Asia (moose, bison, grizzly bear, elk, wolf …)
They had co-evolved with people and were better able to survive
There have been 17 glaciations and one human invasion, yet all the
megafaunal extinctions are correlated with the human invasion
Tim Flannery “the Eternal Frontier”
Human impact on biodiversity
Major extinctions of megafauna occurred on all continents with the
arrival of humans except Africa (where humans evolved)
% p
rehum
an m
agafa
una
Human impact on biodiversity - current rates of extinction
Current rates of extinction are enormous,
rivaling the past 6 major events in terms ofrate and possibly magnitude
Why is extinction so high? - human population growth - climate change - habitat loss (50% landsurface used by humans)
Human impact on biodiversity
Example of bird counts along the
Mississippi and Atlantic flywaysover the last 40 years
4
Why preserve biodiversity?
1. Economic reasons: undiscovered uses of medicinally orcommercially important species, compounds or genes.
Madagascar periwinklecure for cancer
Genes from wildrelatives of cornand other crops
Why preserve biodiversity?
2. Ecological reasons: because of the interdependence of species, loss
of diversity means communities are less productive and less stable
Pro
duct
ivity
(g/m
2/tim
e)
Sta
bili
ty (
% c
hange/tim
e)
Diversity Diversity
more niches full - better exploitation of resources (higher productivity) - less opportunity for invasion
# in
vasi
ons/
time)
Diversity
more redundancy
- bad year for one species might mean good year for others
Why preserve biodiversity?
2. Ecological reasons : Loss of keystonespecies (like the fig) can have particularly
large impacts
Figs are 1% of the plantdiversity but during theseason of fruit scarcity,
80% of the animal biomassdepends on them
Why preserve biodiversity?
3. Ecosystem services : - reducing erosion
Science 2004:
1 sq km of forest inCosta Rica is worth
$40,000 /yr inincreased coffeeproduction to nearby
farms
- purifying water (NYC)
- controlling pests- improve crop pollination
Much recent interest inquantifying benefits from
ecosystem services
Why preserve biodiversity?
4. Esthetic
Why preserve biodiversity?
4. Ethical
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